Electrode Separator

ABSTRACT

A multi-functional battery separator comprises two or more active separator layers deposited from different polymer solutions to form a multilayered unitary structure comprising a free-standing film, a multiplex film on one side of a porous substrate, or separate films or multiplex films on opposite sides of a porous substrate. In a preferred embodiment, the cascade coating method is used to simultaneously deposit the active separator layers wet so that the physical, electrical and morphological changes associated with the polymer drying out process are avoided or minimized. The multi-functional separator is inexpensive to fabricate, exhibits enhanced ionic conductivity and ionic barrier properties, and eliminates gaps between individual layers in a separator stack that can contribute to battery failure.

CROSS-REFERENCE TO RELATED APPLICATION

This PCT Patent Application claims priority to U.S. Provisional PatentApplication Ser. No. 61/040,021, which was filed on Mar. 27, 2008, andis entirely incorporated herein by reference.

FIELD OF THE INVENTION

This invention is concerned with electrical alkaline batteries, and inparticular with separators for alkaline batteries and methods of makingthe same.

BACKGROUND

An electrical storage battery comprises one electrochemical cell or aplurality of electrochemical cells of the same type, the lattertypically being connected in series to provide a higher voltage or inparallel to provide a higher charge capacity than provided by a singlecell. An electrochemical cell comprises an electrolyte interposedbetween and in contact with an anode and a cathode. For a storagebattery, the anode comprises an active material that is readilyoxidized, and the cathode comprises an active material that is readilyreduced. During battery discharge, the anode active material is oxidizedand the cathode active material is reduced, so that electrons flow fromthe anode through an external load to the cathode, and ions flow throughthe electrolyte between the electrodes.

Many electrochemical cells used for electrical storage applications alsoinclude a separator between the anode and the cathode is required toprevent reactants and reaction products present at one electrode fromreacting and/or interfering with reactions at the other electrode. To beeffective, a battery separator must be electronically insulating, andremain so during the life of the battery, to avoid batteryself-discharge via internal shorting between the electrodes. Inaddition, a battery separator must be both an effective electrolytetransport barrier and a sufficiently good ionic conductor to avoidexcessive separator resistance that substantially lowers the dischargevoltage.

Electrical storage batteries are classified as either “primary” or“secondary” batteries. Primary batteries involve at least oneirreversible electrode reaction and cannot be recharged with usefulcharge efficiency by applying a reverse voltage. Secondary batteriesinvolve relatively reversible electrode reactions and can be rechargedwith acceptable loss of charge capacity over numerous charge-dischargecycles. Separator requirements for secondary batteries tend to be moredemanding since the separator must survive repeated charge-dischargecycles.

For secondary batteries comprising a highly oxidative cathode, a highlyreducing anode, and an alkaline electrolyte, separator requirements areparticularly stringent. The separator must be chemically stable instrongly alkaline solution, resist oxidation in contact with the highlyoxidizing cathode, and resist reduction in contact with the highlyreducing anode. Since ions, especially metal oxide ions, from thecathode can be somewhat soluble in alkaline solutions and tend to bechemically reduced to metal on separator surfaces, the separator mustalso inhibit transport and/or chemical reduction of metal ions.Otherwise, a buildup of metal deposits within separator pores canincrease the separator resistance in the short term and ultimately leadto shorting failure due to formation of a continuous metal path throughthe separator. In addition, because of the strong tendency of anodes toform dendrites during charging, the separator must suppress dendriticgrowth and/or resist dendrite penetration to avoid failure due toformation of a dendritic short between the electrodes. A related issuewith anodes is shape change, in which the central part of the electrodetends to thicken during charge-discharge cycling. The causes of shapechange are complicated and not well-understood but apparently involvedifferentials in the current distribution and solution mass transportalong the electrode surface. The separator preferably mitigates zincelectrode shape change by exhibiting uniform and stable ionicconductivity and ionic transport properties.

In order to satisfy the numerous and often conflicting separatorrequirements for zinc-silver oxide batteries, a separator stackcomprised of a plurality of separators that perform specific functionsis needed. Some of the required functions are resistance toelectrochemical oxidation and silver ion transport from the cathode, andresistance to electrochemical reduction and dendrite penetration fromthe anode.

Traditional separators decompose chemically in alkaline electrolytes,which limits the useful life of the battery. Traditional separators arealso subject to chemical oxidation by soluble silver ions andelectrochemical oxidation in contact with silver electrodes.Furthermore, some traditional separators exhibit low mechanical strengthand poor resistance to penetration by dendrites.

To solve some of the problems caused by traditional separators, newseparator materials have been developed.

SUMMARY

The invention provides a multi-functional battery separator comprisingtwo or more active separator layers deposited from different polymersolutions to form a multilayered unitary structure comprising afree-standing film, a multiplex film on one side of a porous substrate,or separate films or multiplex films on opposite sides of a poroussubstrate. In one embodiment, the cascade coating method is used tosimultaneously deposit the active separator layers wet so that thephysical, electrical and morphological changes associated with thepolymer drying out process are avoided or minimized. The activeseparator layers of the multi-functional battery separator of theinvention can also be deposited via conventional methods. The inventionalso provides a process for fabricating the multi-functional batteryseparator.

The multilayered unitary structure of the separator of the inventionprovides better use of the separator active materials, which is believedto improve separator ionic conductivity and effectiveness as an ionictransport barrier. The multilayered unitary structure also reducesbattery production costs and eliminates gaps between individual layersin a separator stack that can contribute to battery failure. Themulti-functional battery separator is particularly useful for batterieswith a zinc anode, for which dendrite formation is an issue, and asilver oxide cathode, which is highly oxidizing. A multi-functionalseparator in this case can comprise a dendrite-resistant separator layerthat faces the anode, and an oxidation-resistant separator layer thatfaces the cathode.

In one aspect, the invention relates to an electrochemical cellcomprising

-   -   an electrolyte,    -   an anode,    -   a cathode, and    -   a multi-functional separator,        wherein the electrolyte is an alkaline electrolyte, the anode        comprises zinc metal, and the multi-functional separator        comprises:    -   an oxidation-resistant separator layer deposited from a PE        solution comprising a polyether polymer that can be linear or        branched and can be unsubstituted or substituted; and    -   a dendrite-resistant separator layer deposited from a PVA        solution comprising a cross-linking agent and a polyvinyl        alcohol precursor polymer, which can be linear or branched and        can be unsubstituted or substituted.

Embodiments of this aspect may include one or more of the followingfeatures. The alkaline electrolyte comprises an aqueous solution of ahydroxide of an alkali metal selected from the group consisting ofpotassium, sodium, lithium, rubidium, cesium, and mixtures thereof. Thecathode comprises an active material selected from the group consistingof silver oxide, nickel oxide, cobalt oxide, and manganese oxide. Thepolyether polymer comprises polyethylene oxide or polypropylene oxide,or a copolymer or a mixture thereof. The cross-linking agent is boricacid. One or both of the PE solution and the PVA solution furthercomprise a powder of a metallic oxide selected from the group consistingof zirconium oxide, titanium oxide and aluminum oxide. One or both ofthe PE solution and the PVA solution further comprise a titanate salt ofan alkali metal selected from the group consisting of potassium, sodium,lithium, rubidium, cesium, and mixtures thereof. One or both of the PEsolution and the PVA solution further comprise a surfactant. The PVAsolution further comprises a plasticizer. The PVA solution furthercomprises a conductivity enhancer consisting of a coploymer of polyvinylalcohol and a hydroxyl-conducting polymer selected from the groupconsisting of polyacrylates, polylactones, polysulfonates,polycarboxylates, polysulfates, polysarconates, polyamides, andpolyamidosulfonates.

In another aspect, the invention features a multi-functional separatorcomprising at least three active separator layers, wherein themulti-functional separator has an ionic resistance of <10 Ω/cm2,electrical resistance of >10 kΩ/cm2, and a wet tensile strength of >0.1lbf.

Embodiments of this aspect of may include one or more of the followingfeatures. The ionic resistance of the separator is <0.5 Ω/cm2. At leasttwo of the three active separator layers comprise a polymeric materialeach individually selected from PVA and PSA, or combinations thereof.The PSA comprises PSS. The multi-functional separator comprises thelayers PVA/V6/PSS; PVA/V6/(PSS+PAA); V6/PVA/(PSS+PAA);PVA/(PSS+PAA(35%))/(PSS+PAA(35%)); (PSS+PAA(35%))/PVA/(PSS+PAA(35%)); or(PSS+PAA (35%))/(PVA(10%)+PSS (20% vs. PVA))/(PSS+PAA (35%)). Themulti-functional separator comprises the layers PVA/V6/(PSS+PAA);V6/PVA/(PSS+PAA); or (PSS+PAA(35%))/PVA/(PSS+PAA (35%)). The separatorthickness is <100 μm. The separator thickness is <30 μm. Each layer inthe separator is <10 μm. The separator impedes dendrite formationrelative to a separator made of the same thickness from PVA. At leasttwo layers of the separator comprise a polymeric material eachindividually selected from PVA, a quaternary ammonium polymer, orcombinations thereof.

In yet another aspect, the invention provides a method of producing aseparator comprising:

-   -   providing a PSA polymer mixture, and    -   providing a PVA polymer mixture,

wherein the PSA polymer mixture and the PVA polymer mixture are providedto form a unitary separator comprising a PSA polymer layer and a PVApolymer layer, wherein the PSA polymer layer resists oxidation and thePVA polymer layer resists dendrite formation.

Embodiments of this aspect may include one or more of the followingfeatures. The separator has a total thickness of less than 100 microns.The method further comprises providing 1 to 10 additional polymermixtures, wherein the polymer mixtures are provided to form a separatorcomprising a PSA polymer layer, a PVA polymer layer, and from 1 to 10additional polymer layers. The separator has an ionic resistance of <10Ω/cm2, electrical resistance of >10 kΩ/cm2, and a wet tensile strengthof >0.1 lbf. The ionic resistance of the separator is <0.5 Ω/cm2.

Further features and advantages of the invention will be apparent tothose skilled in the art from the following detailed description, takentogether with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 depicts a cross-sectional view of a bi-functional separatorhaving two active separator layers deposited on opposite sides of aninert porous substrate film according to the invention;

FIG. 2 depicts a cross-sectional view of a prior art cascade coatingapparatus for producing a free-standing multi-layered film comprisingwet layers;

FIG. 3 depicts a cross-sectional view of a cascade coating apparatusadapted to provide active separator layers on both sides of an optionalsubstrate film;

FIG. 4 illustrates the electrode-separator configuration used for testcells incorporating a bi-functional separator according to theinvention, or analogous individual separator layers;

FIG. 5 shows plots of charge capacity versus cycle number for twozinc-silver oxide cells (A and B), wherein the separator includes aZrO₂—PEO separator layer and two PVA separator layers (70 μm totalthickness); and

FIG. 6 shows plots of charge capacity versus cycle number for twozinc-silver oxide cells (A and B) employing two bi-functionalPVA/ZrO₂—PEO separator layers (60 μm total thickness).

These figures are not to scale and some features have been enlarged forbetter depiction of the features and operation of the invention.Furthermore, these figures are by way of example and are not intended tolimit the scope of the present invention.

DETAILED DESCRIPTION

The invention provides a multi-functional battery separator comprisingtwo or more active separator layers deposited from different polymersolutions to form a multilayered unitary structure comprising afree-standing film, a multiplex film on one side of a porous substrate,or separate films or multiplex films on opposite sides of a poroussubstrate. In a preferred embodiment, the cascade coating method is usedto simultaneously deposit the active separator layers wet so that thephysical, electrical and morphological changes associated with thepolymer drying out process are avoided or minimized. The activeseparator layers of the multi-functional battery separator of theinvention can also be deposited via conventional methods. The inventionalso provides a process for fabricating the multi-functional batteryseparator.

I. DEFINITIONS

The term “battery” encompasses electrical storage devices comprising oneelectrochemical cell or a plurality of electrochemical cells. A“secondary battery” is rechargeable, whereas a “primary battery” is notrechargeable. For secondary batteries of the present invention, abattery anode is designated as the positive electrode during discharge,and as the negative electrode during charge.

The term “alkaline battery” refers to a primary battery or a secondarybattery, wherein the primary or secondary battery comprises an alkalineelectrolyte.

As used herein, a “dopant” or “doping agent” refers to a chemicalcompound that is added to a substance in low concentrations in order toalter the optical/electrical properties of the semiconductor. Forexample, a dopant can be added to the powder active material of acathode to improve its electronic properties (e.g., reduce its impedanceand/or resistivity).

As used herein, an “electrolyte” refers to a substance that behaves asan electrically conductive medium. For example, the electrolytefacilitates the mobilization of electrons and cations in the cell.Electrolytes include mixtures of materials such as aqueous solutions ofalkaline agents. Some electrolytes also comprise additives such asbuffers. For example, an electrolyte comprises a buffer comprising aborate or a phosphate. Exemplary electrolytes include, withoutlimitation aqueous KOH, aqueous NaOH, or the liquid mixture of KOH in apolymer.

As used herein, “alkaline agent” refers to a base or ionic salt of analkali metal (e.g., an aqueous hydroxide of an alkali metal).Furthermore, an alkaline agent forms hydroxide ions when dissolved inwater or other polar solvents. Exemplary alkaline electrolytes includewithout limitation LiOH, NaOH, KOH, CsOH, RbOH, or combinations thereof.

A “cycle” refers to a single charge and discharge of a battery.

For convenience, the polymer name “polyvinylidene fluoride” and itscorresponding initials “PVDF” are used interchangeably as adjectives todistinguish polymers, solutions for preparing polymers, and polymercoatings. Use of these names and initials in no way implies the absenceof other constituents. These adjectives also encompass substituted andco-polymerized polymers. A substituted polymer denotes one for which asubstituent group, a methyl group, for example, replaces a hydrogen onthe polymer backbone.

For convenience, the polymer name “polytetrafluoroethylene” and itscorresponding initials “PTFE” are used interchangeably as adjectives todistinguish polymers, solutions for preparing polymers, and polymercoatings. Use of these names and initials in no way implies the absenceof other constituents. These adjectives also encompass substituted andco-polymerized polymers. A substituted polymer denotes one for which asubstituent group, a methyl group, for example, replaces a hydrogen onthe polymer backbone.

As used herein, “Ah” refers to Ampere (Amp) Hour and is a scientificunit for the capacity of a battery or electrochemical cell. A derivativeunit, “mAh” represents a milliamp hour and is 1/1000 of an Ah.

As used herein, “maximum voltage” or “rated voltage” refers to themaximum voltage an electrochemical cell can be charged withoutinterfering with the cell's intended utility. For example, in severalzinc-silver electrochemical cells that are useful in portable electronicdevices, the maximum voltage is less than about 3.0 V (e.g., less thanabout 2.8 V, less than about 2.5 V, about 2.3 V or less, or about 2.0V). In other batteries, such as lithium ion batteries that are useful inportable electronic devices, the maximum voltage is less than about 15.0V (e.g., less than about 13.0 V, or about 12.6 V or less). The maximumvoltage for a battery can vary depending on the number of charge cyclesconstituting the battery's useful life, the shelf-life of the battery,the power demands of the battery, the configuration of the electrodes inthe battery, and the amount of active materials used in the battery.

As used herein, an “anode” is an electrode through which (positive)electric current flows into a polarized electrical device. In a batteryor galvanic cell, the anode is the negative electrode from whichelectrons flow during the discharging phase in the battery. The anode isalso the electrode that undergoes chemical oxidation during thedischarging phase. However, in secondary, or rechargeable, cells, theanode is the electrode that undergoes chemical reduction during thecell's charging phase. Anodes are formed from electrically conductive orsemiconductive materials, e.g., metals, metal oxides, metal alloys,metal composites, semiconductors, or the like. Common anode materialsinclude Si, Sn, Al, Ti, Mg, Fe, Bi, Zn, Sb, Ni, Pb, Li, Zr, Hg, Cd, Cu,LiC₆, mischmetals, alloys thereof, oxides thereof, or compositesthereof.

Anodes can have many configurations. For example, an anode can beconfigured from a conductive mesh or grid that is coated with one ormore anode materials. In another example, an anode can be a solid sheetor bar of anode material.

As used herein, a “cathode” is an electrode from which (positive)electric current flows out of a polarized electrical device. In abattery or galvanic cell, the cathode is the positive electrode intowhich electrons flow during the discharging phase in the battery. Thecathode is also the electrode that undergoes chemical reduction duringthe discharging phase. However, in secondary or rechargeable cells, thecathode is the electrode that undergoes chemical oxidation during thecell's charging phase. Cathodes are formed from electrically conductiveor semiconductive materials, e.g., metals, metal oxides, metal alloys,metal composites, semiconductors, or the like. Common cathode materialsinclude AgO, Ag₂O, HgO, Hg₂O, CuO, CdO, NiOOH, Pb₂O₄, PbO₂, LiFePO₄,Li₃V₂(PO₄)₃, V₆O₁₃, V₂O₅, Fe₃O₄, Fe₂O₃, MnO₂, LiCoO₂, LiNiO₂, LiMn₂O₄,or composites thereof.

Cathodes can also have many configurations. For example, a cathode canbe configured from a conductive mesh that is coated with one or morecathode materials. In another example, a cathode can be a solid sheet orbar of cathode material.

As used herein, an “electronic device” is any device that is powered byelectricity. For example, and electronic device can include a portablecomputer, a portable music player, a cellular phone, a portable videoplayer, or any device that combines the operational features thereof.

As used herein, “cycle life” is the maximum number of times a secondarybattery can be charged and discharged.

The symbol “M” denotes molar concentration.

Batteries and battery electrodes are denoted with respect to the activematerials in the fully-charged state. For example, a zinc-silver oxidebattery comprises an anode comprising zinc and a cathode comprisingsilver oxide. Nonetheless, more than one species is present at a batteryelectrode under most conditions. For example, a zinc electrode generallycomprises zinc metal and zinc oxide (except when fully charged), and asilver oxide electrode usually comprises silver oxide (AgO and/or Ag₂O)and silver metal (except when fully discharged).

The term “oxide” applied to alkaline batteries and alkaline batteryelectrodes encompasses corresponding “hydroxide” species, which aretypically present, at least under some conditions.

As used herein “substantially stable” or “substantially inert” refers toa compound or component that remains substantially chemically unchangedin the presence of an alkaline electrolyte (e.g., potassium hydroxide)and/or in the presence of an oxidizing agent (e.g., silver ions presentin the cathode or dissolved in the electrolyte).

As used herein, “charge profile” refers to a graph of an electrochemicalcell's voltage or capacity with time. A charge profile can besuperimposed on other graphs such as those including data points such ascharge cycles or the like.

As used herein, “resistivity” or “impedance” refers to the internalresistance of a cathode in an electrochemical cell. This property istypically expressed in units of Ohms or micro-Ohms.

As used herein, the terms “first” and/or “second” do not refer to orderor denote relative positions in space or time, but these terms are usedto distinguish between two different elements or components. Forexample, a first separator does not necessarily proceed a secondseparator in time or space; however, the first separator is not thesecond separator and vice versa. Although it is possible for a firstseparator to proceed a second separator in space or time, it is equallypossible that a second separator proceeds a first separator in space ortime.

For convenience, both polymer names “polyether”, “polyethylene oxide”,“polypropylene oxide” and “polyvinyl alcohol” and their correspondinginitials “PE”, “PEO”, “PPO” and “PVA”, respectively, are usedinterchangeably as adjectives to distinguish polymers, solutions forpreparing polymers, and polymer coatings. Use of these names andinitials in no way implies the absence of other constituents. Theseadjectives also encompass substituted and co-polymerized polymers. Asubstituted polymer denotes one for which a substituent group, a methylgroup, for example, replaces a hydrogen on the polymer backbone.

As used herein “oxidation-resistant” refers to a separator that resistsoxidation in an electrochemical cell of an alkaline battery and/or issubstantially stable in the presence of an alkaline electrolyte and/oran oxidizing agent (e.g., silver ions).

As used herein, a “titanate salt” refers to a chemical salt thatincludes in its chemical formula TiO₃. Examples of titanate saltsinclude potassium titanate, sodium titanate, lithium titanate, rubidiumtitanate, or cesium titanate, without limitation.

As used herein, “adjacent” refers to the positions of at least twodistinct elements (e.g., at least one separator and at least oneelectrode (e.g., an anode and/or a cathode)). When an element such as aseparator is adjacent to another element such as an electrode or even asecond separator, one element is positioned to contact or nearly contactanother element. For example, when a separator is adjacent to anelectrode, the separator electrically contacts the electrode when theseparator and electrode are in an electrolyte environment such as theenvironment inside an electrochemical cell. The separator can be inphysical contact or the separator can nearly contact the electrode suchthat any space between the separator and the electrode is void of anyother separators or electrodes. It is noted that electrolyte can bepresent in any space between a separator that is adjacent to anelectrode or another separator.

As used herein, “unitary structure” refers to a structure that includesone or more elements that are concurrently or almost concurrentlyprocessed to form the structure. One noteworthy characteristic of manyunitary structures is the presence of two domains at the interfacebetween two elements. For example, an electrochemical cell separatorthat is a unitary structure is one in which all of the separatoringredients or starting materials concurrently undergo a process (otherthan mechanical combination) that combines them and forms a singleseparator. For instance, a separator that includes a plurality of layersthat are formed by coextruding the starting materials from a pluralityof sources generates a unitary structure, wherein the interface betweenlayers includes domains of each layer that terminates at the interface.This unitary structure is not equivalent to a separator that includes aplurality of layers that are each individually formed and mechanicallystacked to form a multi-layered separator.

The interface between layers in a unitary structure contains domains ofeach layer that joins at the interface, such that the interfacecomprises both layers simultaneously. This property is characteristic ofunitary structures.

As used herein “dendrite-resistant” refers to a separator that reducesthe formation of dendrites in an electrochemical cell of an alkalinebattery under normal operating conditions, i.e., when the batteries arestored and used in temperatures from about −20° C. to about 70° C., andare not overcharged or charged above their rated capacity and/or issubstantially stable in the presence of an alkaline electrolyte, and/oris substantially stable in the presence of a reducing agent (e.g., ananode comprising zinc). In some examples, a dendrite-resistant separatorinhibits transport and/or chemical reduction of metal ions.

II. SEPARATORS

Separators of the present invention comprise a unitary structure formedfrom at least two strata or layers. The separator can include layerswherein each layer comprises the same material, or each layer comprisesa different material; or, the strata are layered to provide layers ofthe same material and at least on layer of another material. In severalembodiments, one stratum comprises an oxidation resistant material, andthe remaining stratum comprises a dendrite resistant material. In otherembodiments, at least one layer comprises an oxidation-resistantmaterial, or at least one layer comprises a dendrite-resistant material.The unitary structure is formed when the material comprising one layer(e.g., an oxidation-resistant material) is coextruded with the materialcomprising another layer (e.g., a dendrite resistant material oroxidation-resistant material). In several embodiments, the unitaryseparator is formed from the coextrusion of oxidation-resistant materialwith dendrite-resistant material.

In several embodiments, the oxidation-resistant material comprises apolyether polymer mixture and the dendrite resistant material comprisesa PVA polymer mixture. In another example, the dendrite-resistantseparator layer for use in a multi-functional separator for an alkalinezinc-silver oxide battery comprises a cross-linked polyvinyl alcohol(PVA) film deposited from a PVA solution comprising a cross-linkingagent and a polyvinyl alcohol precursor polymer, which can be linear orbranched and can be unsubstituted or substituted. In several examples,the PVA precursor polymer is at least 80% hydrolyzed and has an averagemolecular weight in the range from 150,000 to 190,000.

It is noted that separators useful in electrochemical cells can beconfigured in any suitable way such that the separator is substantiallyinert in the presence of the anode, cathode and electrolyte of theelectrochemical cell. For example, a separator for a rectangular batteryelectrode can be in the form of a sheet or film comparable in size orslightly larger than the electrode, and can simply be placed on theelectrode or can be sealed around the edges. The edges of the separatorcan be sealed to the electrode, an electrode current collector, abattery case, or another separator sheet or film on the backside of theelectrode via an adhesive sealant, a gasket, or fusion (heat sealing) ofthe separator or another material. The separator can also be in the formof a sheet or film wrapped and folded around the electrode to form asingle layer (front and back), an overlapping layer, or multiple layers.For a cylindrical battery, the separator can be spirally wound with theelectrodes in a jelly-roll configuration. Typically, the separator isincluded in an electrode stack comprising a plurality of separators. Theoxidation-resistant separator of the invention can be incorporated in abattery in any suitable configuration.

In many embodiments, separators of the present invention comprise activeseparator layers formulated from an ionic conducting polymer material(polyvinyl alcohol or polyethylene oxide, for example) and can include ametallic oxide filler material (zirconium oxide, titanium oxide oraluminum oxide, for example). Although not wishing to be limited bytheory, it is theorized that the filler material impedes transport ofdetrimental ions (silver and zinc ions in zinc-silver oxide battery).Active separator layers can also include a conductivity enhancer(inorganic or organic), a surfactant, and/or a plasticizer.

The invention provides a multi-functional battery separator comprising aplurality of active separator layers that form a multilayered unitarystructure. Each of the active separator layers is deposited from aseparate solution or mixture. In some embodiments, each solution ormixture has a different composition. In others, at least two of theseparate mixtures or solutions have about the same composition.

In one embodiment, at least two of the active separator layers aresimultaneously deposited wet by the cascade coating method, either as afree-standing multi-functional separator or as multi-functional coatingson a porous substrate film. In this case, physical, electrical andmorphological changes associated with the polymer drying out process areavoided or minimized. In another embodiment, at least one of the activeseparator layers is deposited as a free-standing film and at least oneother active separator layer is deposited thereupon.

The multi-functional separator of the invention can further comprise aporous or nonporous substrate film, on which at least one of the activeseparator layers is deposited. In this case, the multi-functionalseparator can comprise a multiplex film on one side of a poroussubstrate, or separate films or multiplex films on opposite sides of aporous substrate. The invention also provides a process for fabricatingthe multi-functional battery separator.

FIG. 1 depicts a cross-sectional view of a bi-functional separator 100comprising two active separator coatings 101 and 102 deposited onopposite sides of an inert porous substrate film 105 according to theinvention. Active materials 103 and 104 from coatings 101 and 102,respectively, have fully penetrated within the pores of substrate film105. At least some penetration of active materials 103 and 104 withinthe pores of substrate film 105 is preferred but can be less than fullpenetration.

Active separator layers for the multi-functional separator of theinvention can be applied by any suitable method. Methods for formingfree-standing multi-functional separators or for applying separatorlayer coatings to a porous substrate include those selected from thegroup consisting of pouring, spreading, casting, pressing, backfilling,dipping, spraying, rolling, laminating, extruding, and combinationsthereof.

In one embodiment, the method of forming the multi-functional separatorof the invention is the cascade coating method, which can be used toform free-standing multi-functional separators, or to apply multipleactive separator layers to a porous substrate film. In the cascadecoating method, liquid or gelled solutions, each containing theconstituents of a given layer, are co-extruded in sheets that flowtogether to simultaneously form the multi-layered structure.

FIG. 2 depicts a cross-sectional view of a one exemplary cascade coatingapparatus 200 for producing a free-standing multi-layered filmcomprising two wet layers. Solution 201 contained in reservoir 203 isflowed through slot 205 and forms film 207, while solution 202 containedin reservoir 204 is flowed through slot 206 and forms film 208 disposedupon film 207. Solutions 201 and 202 can be caused to flow by anysuitable means, including gravity (as shown), gas pressure, or a pump(not shown). The extrusion rate (adjusted via solution viscosity andflow pressure) and solvent evaporation rate (adjusted via solutioncomposition, temperature and humidity) are optimized to provide solid orsemi-solid films of a desired consistency and wetness. As those in theart will appreciate, this apparatus can be readily expanded to formmulti-functional separators having more than two layers.

Inter-diffusion and intermixing of the components of films 207 and 208can be minimized by adjusting the solution viscosities, extrusion ratesand solvent evaporation rates, and by applying heat to the extrudedmulti-layered film via radiant heating or forced convection heating.Alternative solvents and surfactant additives for solutions 201 and 202can also be used to render one adjacent separator layer hydrophobic andthe other adjacent layer hydrophilic. Polar surfactants having ahydrophobic head and a hydrophilic tail can be especially efficacious

FIG. 3 depicts a cross-sectional view of a cascade coating apparatus 300adapted to provide active separator layers on both sides of a poroussubstrate film. Solution 301 contained in reservoir 303 is flowedthrough slot 305 and forms active separator film 307, while solution 302contained in reservoir 304 is flowed through slot 306 and forms activeseparator film 308, wherein both active separator film 307 and activeseparator film 308 are disposed on porous substrate film 309 as itpasses between the openings in slots 305 and 306. Porous substrate film309 can be conveyed via rollers 310 a-d of a conveyor. Active separatorfilms 307 and 308 can penetrate into pores in porous substrate filmpartially (as shown) or fully.

One exemplary separator of the present invention includes 3 layers. Forinstance a first layer is a hydrophilic polymer dense film system. Thissystem can be prepared by co-extruding an aqueous polymer solution ontoa second layer. The aqueous polymer solution can be prepared bydissolving the polymer in water at 5-20 wt %. Exemplary polymers includepolyethylene oxide, polyethylene glycol, polyvinyl alcohol, or polyvinylalcohol copolymers. Polymer formulations can optionally include inertfillers, ion exchanging fillers, soluble fillers, plasticizers,extractable (immiscible) phase segregating liquids and copolymers withboth hydrophobic and hydrophilic sub units, i.e., PEO-PMMA copolymers.

The second layer of this exemplary separator can be a hydrophiliccomposite film system. This system can be prepared by co-extruding anaqueous composite mixture between the first layer and a third layer. Theaqueous composite mixture can be prepared by dispersing metal oxideparticles in an aqueous polymer solution. In one example, the metaloxide particles are zirconium dioxide, titanium dioxide, or combinationsthereof. Exemplary polymers useful for this second layer includepolyethylene oxide, polyethylene glycol, polyvinyl alcohol, or polyvinylalcohol copolymers. The solid concentration of the aqueous compositemixture ranges from 10 to 40 wt %. The metal oxide particles to polymerweight ratio ranges from 0.5 to 5. The formulation can also have inertfillers, ion exchanging fillers, soluble fillers, plasticizers,extractable (immiscible) phase segregating liquids and copolymers withboth hydrophobic and hydrophilic sub units, i.e., PEO-PMMA copolymers.

Also, in this exemplary separator a third later can be a hydrophilicpolymer dense film system. One exemplary system is prepared byco-extruding an aqueous polymer solution between the second layer and anoptional substrate. The aqueous polymer solution can be prepared bydissolving the polymer in water at 5-20 weight percent. The polymer canconsist of polyethylene oxide, polyethylene glycol, polyvinyl alcohol orpolyvinyl alcohol copolymers. The formulation can also have inertfillers, ion exchanging fillers, soluble fillers, plasticizers,extractable (immiscible) phase segregating liquids and copolymers withboth hydrophobic and hydrophilic sub units, i.e., PEO-PMMA copolymers.

In several layered separators of the present invention, the layers canhave the same composition or different compositions. For instance, inthree layered separator described above, two of the layers can includethe same composition, or each of the three layers can comprise differentcompositions. The only restrictions on the layer order are that they cannot mix after co-extrusion but before they dry.

Exemplary processing conditions for the exemplary 3-layer separatorsystem is prepared by simultaneously co-extruding 3 individual aqueousmixtures onto a carrier substrate using a triple-layer slot die. Thecast 3-layer film is dried at about 180 degrees Celsius in an 18-footconvection oven under about 1 foot per minute line speed. The die gapsare set in various combinations to achieve a total dried film thicknessof about 50 to 150 micrometers.

Exemplary materials for the substrate include polypropylene, hydrophilicnon-woven polyolefins, polyesters, polyamides, perfluorinated polymers,or polysulfones.

The 3-layered separators described above are relatively environmentallyfriendly. The use of other solvents is possible and some times preferredas when you want to prepare a layer by phase inversion. An example wouldbe PVDF:HFP in acetone when coextruded with an aqueous layer would causethe PVDF:HFP to exit the solution in a highly porous network.

The multi-functional separator of the invention can be used with anybattery, comprising any electrolyte, any anode and any cathode. Theinvention is especially suitable for use in an alkaline storage batterycomprising a zinc anode and a silver oxide cathode, but can be used withother anodes and other cathodes. The invention can be used with anodescomprising zinc, cadmium or mercury, or mixtures thereof, for example,and with cathodes comprising silver oxide, nickel oxide, cobalt oxide ormanganese oxide, or mixtures thereof, for example.

A. Polyether Polymer Material

In several embodiments of the present invention the oxidation-resistantlayer of the separator comprises a polyether polymer material that iscoextruded with a dendrite-resistant material. The polyether materialcan comprise polyethylene oxide (PEO) or polypropylene oxide (PPO), or acopolymer or a mixture thereof. The polyether material can also becopolymerized or mixed with one or more other polymer materials,polyethylene, polypropylene and/or polytetrafluoroethylene (PTFE), forexample. In some embodiments, the PE material is capable of forming afree-standing polyether film when extruded alone, or can form a freestanding film when coextruded with a dendrite-resistant material.Furthermore, the polyether material is substantially inert in thealkaline battery electrolyte and in the presence of silver ions.

In alternative embodiments, the oxidation resistant material comprises aPE mixture that optionally includes zirconium oxide powder. Withoutintending to be limited by theory, it is theorized that the zirconiumoxide powder inhibits silver ion transport by forming a surface complexwith silver ions. The term “zirconium oxide” encompasses any oxide ofzirconium, including zirconium dioxide and yttria-stabilized zirconiumoxide. The zirconium oxide powder is dispersed throughout the PEmaterial so as to provide a substantially uniform silver complexationand a uniform barrier to transport of silver ions. In severalembodiments, the average particle size of the zirconium oxide powder isin the range from about 1 nm to about 5000 nm, e.g., from about 5 nm toabout 100 nm.

In other embodiments, the oxidation-resistant material further comprisesan optional conductivity enhancer. The conductivity enhancer cancomprise an inorganic compound, potassium titanate, for example, or anorganic material. Titanates of other alkali metals than potassium can beused. Suitable organic conductivity enhancing materials include organicsulfonates and carboxylates. Such organic compounds of sulfonic andcarboxylic acids, which can be used singly or in combination, comprise awide range of polymer materials that can include salts formed with awide variety of electropositive cations, K⁺, Na⁺, Li⁺, Pb⁺², Ag⁺, NH4⁺,Ba⁺², Sr⁺², Mg⁺², Ca⁺² or anilinium, for example. These compounds alsoinclude commercial perfluorinated sulfonic acid polymer materials,Nafion® and Flemion®, for example. The conductivity enhancer can includea sulfonate or carboxylate copolymer, with polyvinyl alcohol, forexample, or a polymer having a 2-acrylamido-2-methyl propanyl as afunctional group. A combination of one or more conductivity enhancingmaterials can be used.

Oxidation-resistant material that is coextruded to form a separator ofthe present invention can comprise from about 5 wt % to about 95 wt %(e.g., from about 20 wt % to about 60 wt %, or from about 30 wt % toabout 50 wt %) of zirconium oxide and/or conductivity enhancer.

Oxidation-resistant materials can also comprise additives such assurfactants that improve dispersion of the zirconium oxide powder bypreventing agglomeration of small particles. Any suitable surfactant canbe used, including one or more anionic, cationic, non-ionic, ampholytic,amphoteric and zwitterionic surfactants, and mixtures thereof. In oneembodiment, the separator comprises an anionic surfactant. For example,the separator comprises an anionic surfactant, and the anionicsurfactant comprises a salt of sulfate, a salt of sulfonate, a salt ofcarboxylate, or a salt of sarcosinate. One useful surfactant comprisesp-(1,1,3,3-tetramethylbutyl)-phenyl ether, which is commerciallyavailable under the trade name Triton X-100 from Rohm and Haas.

In several embodiments, the oxidation-resistant material comprises fromabout 0.01 wt % to about 1 wt % of surfactant.

In another embodiment, the oxidation-resistant separator layer comprisesa polyether (PE) film deposited from a PE solution comprising apolyether polymer that can be linear or branched and can beunsubstituted or substituted. For example, the polyether polymercomprises a linear or branched polyethylene oxide (PEO) or polypropyleneoxide (PPO), or a copolymer or a mixture thereof. The polyether materialcan comprise a copolymer or a mixture of the polyether polymer with oneor more polymer materials other than a polyether, for example,polyethylene, polypropylene, polyphenylene oxide, polysulfone,acrylonitrile butadiene styrene (ABS), or polytetrafluoroethylene.Primary requirements are that the polyether film be substantially inertin the alkaline battery electrolyte and in the presence of silver ions.Another exemplary polyether polymer is polyethylene oxide such as thosehaving an average molecular weight in the range 0.5 to 5.0 million.

The PE solution can also comprises a powder of a metallic oxide,zirconium oxide, titanium oxide or aluminum oxide, for example, as afiller to more effectively block transport of silver ions. One exemplarymetallic oxide filler is zirconium oxide, which is thought to inhibitssilver ion transport by forming a surface complex with silver ions. Inseveral examples, the powder of zirconium oxide (or other metallicoxide) is well dispersed throughout the PE film so as to provide auniform barrier to transport of silver ions. The average particle sizeof the zirconium oxide powder (or other metallic oxide powder) should bein the range from 1 to 5000 nm, preferably in the range from 5 to 200nm. Zirconium oxide filler tends to increase the ionic conductivity ofthe oxidation-resistant separator layer.

In one embodiment, the concentrations in weight percent of thecomponents in a the PE solution are within the ranges: 87 to 95% water;2 to 6% polyethylene oxide (PE polymer); 2 to 6% yttria-stabilizedzirconium oxide (filler); 0.2 to 1.5% potassium titanate (conductivityenhancer); and 0.08 to 0.2% Triton X-100 (surfactant). These ranges canbe adjusted for different PE polymers, fillers, conductivity enhancers,and surfactants.

B. Polyvinyl Polymer Material

In several embodiments of the present invention the dendrite-resistantstratum of the separator comprises a polyvinyl polymer material that iscoextruded with the oxidation-resistant material. In severalembodiments, the PVA material comprises a cross-linked polyvinyl alcoholpolymer and a cross-linking agent.

In several embodiments, the cross-linked polyvinyl alcohol polymer is acopolymer. For example, the cross-linked PVA polymer is a copolymercomprising a first monomer, PVA, and a second monomer. In someinstances, the PVA polymer is a copolymer comprising at least 60 molepercent of PVA and a second monomer. In other examples, the secondmonomer comprises vinyl acetate, ethylene, vinyl butyral, or anycombination thereof.

PVA material useful in separators of the present invention also comprisea cross-linking agent in a sufficient quantity as to render theseparator substantially insoluble in water. In several embodiments, thecross-linking agent used in the separators of the present inventioncomprises a monoaldehyde (e.g., formaldehyde or glyoxilic acid);aliphatic, furyl or aryl dialdehydes (e.g., glutaraldehyde, 2,6furyldialdehyde or terephthaldehyde); dicarboxylic acids (e.g., oxalicacid or succinic acid); polyisocyanates; methylolmelamine; copolymers ofstyrene and maleic anhydride; germaic acid and its salts; boroncompounds (e.g., boron oxide, boric acid or its salts; or metaboric acidor its salts); or salts of copper, zinc, aluminum or titanium. Forexample, the cross-linking agent comprises boric acid.

In another embodiment, the PVA material optionally comprises zirconiumoxide powder. In several embodiments, the PVA material comprises fromabout 1 wt % to about 99 wt % (e.g., from about 2 wt % to about 98 wt %,from about 20 wt % to about 60 wt %, or from about 30 wt % to about 50wt %).

In many embodiments, the dendrite-resistant strata of the separator ofthe present invention comprises a reduced ionic conductivity. Forexample, in several embodiments, the separator comprises an ionicresistance of less than about 20 mΩ/cm², (e.g., less than about 10mΩ/cm², less than about 5 mΩ/cm², or less than about 4 mΩ/cm²).

The PVA material that forms the dendrite-resistant stratum of theseparator of the present invention can optionally comprise any suitableadditives such as a conductivity enhancer, a surfactant, a plasticizer,or the like.

In some embodiments, the PVA material further comprises a conductivityenhancer. For example, the PVA material comprises a cross-linkedpolyvinyl alcohol polymer, a zirconium oxide powder, and a conductivityenhancer. The conductivity enhancer comprises a copolymer of polyvinylalcohol and a hydroxyl-conducting polymer. Suitable hydroxyl-conductingpolymers have functional groups that facilitate migration of hydroxylions. In some examples, the hydroxyl-conducting polymer comprisespolyacrylate, polylactone, polysulfonate, polycarboxylate, polysulfate,polysarconate, polyamide, polyamidosulfonate, or any combinationthereof. A solution containing a copolymer of a polyvinyl alcohol and apolylactone is sold commercially under the trade name Vytek® polymer byCelanese, Inc. In several examples, the separator comprises from about 1wt % to about 10 wt % of conductivity enhancer.

In other embodiments, the PVA material further comprises a surfactant.For example, the separator comprises a cross-linked polyvinyl alcoholpolymer, a zirconium oxide powder, and a surfactant. The surfactantcomprises one or more surfactants selected from an anionic surfactant, acationic surfactant, a nonionic surfactant, an ampholytic surfactant, anamphoteric surfactant, and a zwitterionic surfactant. Such surfactantsare commercially available. In several examples, the PVA materialcomprises from about 0.01 wt % to about 1 wt % of surfactant.

In several embodiments, the dendrite-resistant stratum further comprisesa plasticizer. For example, the dendrite-resistant stratum comprises across-linked polyvinyl alcohol polymer, a zirconium oxide powder, and aplasticizer. The plasticizer comprises one or more plasticizers selectedfrom glycerin, low-molecular-weight polyethylene glycols, aminoalcohols,polypropylene glycols, 1,3 pentanediol branched analogs, 1,3pentanediol, and/or water. For example, the plasticizer comprisesgreater than about 1 wt % of glycerin, low-molecular-weight polyethyleneglycols, aminoalcohols, polypropylene glycols, 1,3 pentanediol branchedanalogs, 1,3 pentanediol, or any combination thereof, and less than 99wt % of water. In other examples, the plasticizer comprises from about 1wt % to about 10 wt % of glycerin, low-molecular-weight polyethyleneglycols, aminoalcohols, polypropylene glycols, 1,3 pentanediol branchedanalogs, 1,3 pentanediol, or any combination thereof, and from about 99wt % to about 90 wt % of water.

In some embodiments, the separator of the present invention furthercomprises a plasticizer. In other examples, the plasticizer comprisesglycerin, a low-molecular-weight polyethylene glycol, an aminoalcohol, apolypropylene glycols, a 1,3 pentanediol branched analog, 1,3pentanediol, or combinations thereof, and/or water.

The cross-linked polyvinyl alcohol polymer can be a copolymer comprisinga copolymerized polymer and at least 60 mole percent polyvinyl alcohol.The copolymer is formed by including the monomer of the copolymerizedpolymer in the PVA solution. Suitable monomers for forming a PVAcopolymer include vinyl acetate, ethylene, vinyl butyral, and mixturesthereof.

Cross-linking is necessary to render the polyvinyl alcohol polymerinsoluble in water. Suitable cross-linking agents that can be added tothe PVA solution to effect cross-linking of the polyvinyl alcoholprecursor polymer include monoaldehydes (formaldehyde and glyoxilicacid, for example), aliphatic, furyl or aryl dialdehydes(glutaraldehyde, 2,6 furyldialdehyde and terephthaldehyde, for example),dicarboxylic acids (oxalic acid and succinic acid, for example),polyisocyanates, methylolmelamine, copolymers of styrene and maleicanhydride, germaic acid and its salts, boron, compounds (boron oxide,boric acid and its salts, and metaboric acid and its salts, forexample), and salts of copper, zinc, aluminum and titanium. A preferredcross-linking agent is boric acid.

In a preferred embodiment, the PVA solution further comprises a powderof an insoluble metallic oxide, zirconium oxide, titanium oxide oraluminum oxide, for example, as a filler material to more effectivelyblock transport of silver and zinc ions and suppress growth of zincdendrites. A preferred filler material is zirconium oxide powder, asdescribed in paragraph [0034] for the oxidation-resistant separatorlayer.

In one embodiment, the concentrations in weight percent of thecomponents in a PVA solution are 95% water, 3.1% polyvinyl alcohol(average molecular weight of 150,000), 1.9% zirconium oxide (ZrO₂ of 0.6μm average particle size), and 0.06% boric acid.

C. PolySulfonic Acid (PSA) Polymer Material

In another aspect, the present invention provides a multilayered batteryseparator for use in an alkaline electrochemical cell. The separatorincludes a PSA polymer material.

It is noted that in multilayered separators of the present invention,the layers can be stacked in any order.

The PSA polymer material comprises PSA, which can be present as a PSAhomopolymer, a PSA copolymer (e.g., a block copolymer, a randomcopolymer, an alternating copolymer, or the like), or a mixture of PSAhomopolymer or a PSA copolymer and another polymer or copolymer.

In several embodiments, the PSA polymer material comprises a mixture ofPSA (e.g., polystyrene sulfonic acid (PSS) or other polysulfonic acid offormula I) homopolymer or a PSA copolymer and another polymer orcopolymer. For example, the PSA polymer material comprises a mixture ofPSA (e.g., polystyrene sulfonic acid or other polysulfonic acid offormula I) and polyacrylic acid (e.g., polymethylacrylic acid, acrylicacid grafted fluorinated polymer, or the like), acrylic acid copolymer,polyacrylamide, acrylamide copolymer, polyvinyl amine, vinyl aminecopolymer, maleic acid copolymer, maleic anhydride copolymer, polyvinylether, vinyl ether copolymer, polyethylene glycol, ethylene glycolcopolymer, polypropylene glycol, polypropylene glycol copolymer,sulfonated polysulfone, sulfonated polyethersulfone, sulfonatedpolyetheretherketone, polyallyl ether (e.g., polyvinyl ethet),polydivinylbenzene, or triallyltriazine.

In other embodiments, the PSA polymer material comprises polystyrenesulfonic acid homopolymer.

PSA polymer material can also comprise one or more optional additivessuch as surfactants, plasticizers, fillers, combinations thereof, or thelike, such as those described above.

D. Quaternary Ammonium Polymers

The multilayer separator may also include a quaternary ammonium polymer.A quaternary ammonium polymer includes any polymer including aquaternary nitrogen. Examples of quaternary ammonium polymers include,but are not limited, poly[(2-ethyldimethylammonioethyl methacrylateethyl sulfate)-co-(1-vinylpyrrolidone)], a homopolymer ofpoly(2-dimethylamino)ethyl methacrylate) methyl chloride quaternarysalt, poly(acrylamide-co-diallyldimethylammonium chloride), homopolymerof Polymer3: poly(diallyldimethylammonium chloride),poly(dimethylamine-co-epichlorohydrin-co-ethylenediamine) or mixturesthereof.

E. Optional Substrate

In alternative embodiments, the separator of the present battery furthercomprises a substrate on which polymer materials (e.g.,oxidation-resistant material and/or dendrite-resistant material) arecoextruded. In some examples, the separate polymer materials arecoextruded onto a single surface of the substrate. In other examples,the polymer materials are coextruded onto opposing surfaces of thesubstrate such that at least two strata forming the separator areseparated by the substrate.

Substrates useful in these novel separators can comprise any suitablematerial that is substantially inert in an alkaline electrochemicalcell. In several embodiments, the substrate is a woven or non-wovensheet. In other embodiments, the substrate is a non-woven sheet.

The substrate film can comprise any suitable organic polymer orinorganic material that is electronically insulating, providessufficient structural integrity, and is chemically and electrochemicallystable in concentrated alkaline solutions. Some suitable organic polymermaterials comprise polyolefins (polyethylene or polypropylene, forexample), polyethers (polyethylene oxide and polypropylene oxide, forexample), polyfluorocarbons (polytetrafluoroethylene, for example),polyamides (nylon, for example), polysulfones (Udel® sold by Solvay, forexample), polyethersulfones (Radel® sold by Solvay, for example),polyacrylates, polymethacrylates, polystyrenes, and mixtures,co-polymers and substituted polymers thereof. Porous films of commercialblended polymers, ABS (acrylonitrile butadiene styrene) or EPDM(ethylene-propylene-diene terpolymer), for example, can be used.Suitable inorganic materials include metallic oxides, including aluminumoxide, titanium oxide, zirconium oxide, yttria-stabilized zirconiumoxide, and mixtures thereof, and metallic nitrides, including titaniumnitride, aluminum nitride, zirconium nitride, and mixtures thereof.

As mentioned above, separators of the present invention can include anynumber of layers and can have any thickness; however, due to specialconsiderations of electrochemical cell housings, it is desired tomaximize the number of layers of the separator and minimize the overallthickness of the separator.

In many embodiments, the separator comprises a total thickness of lessthan 200 microns (e.g., less than 150 microns or less than 100 microns).In other embodiments, the separator comprises from 2 to 20 layers (e.g.,from 2 to 15 layers, from 2 to 10 layers, or from 2 to 5 layers. Inother examples, the separator has a total thickness of less than 200microns and comprises from about 2 to about 20 layers.

III. CO-EXTRUDED SEPARATOR PROPERTIES

As described herein, the invention provides a multi-functional batteryseparator comprising two or more active separator layers deposited fromdifferent polymer solutions to form a multilayered unitary structurecomprising a free-standing film, a multiplex film on one side of aporous substrate, or separate films or multiplex films on opposite sidesof a porous substrate. The separator can be fashioned to resist bothoxidation and dendrite formation.

In one embodiment, a separator providing resistance to both oxidationand dendrite formation may posses three basic properties (three primaryproperties): ionic resistance acceptable for the application (<0.5 Ω/cm2for high discharge rate applications or <10 Ω/cm2 for low discharge rateapplications), high electrical resistance (>10 kΩ/cm2), and wet tensilestrength (>0.1 lbf). In other embodiments, additional properties of theseparator (secondary properties), such as chemical resistance,differential affinity for different ions present in the cell,sequestration of certain chemical species present in the electrolyte, orlayers or surfaces that are more compliant or gel like, may be useful.However, many of the materials that impart these properties oftendiminish one or more of the three primary properties. For example, thereare many materials with specialized chemical resistance that alsopossesses high ionic resistivity or materials with very low ionicresistance that exhibit poor tensile strength. The current inventionallows for materials with chemical resistance but high ionic resistanceto be combined in a thin layer with a thicker layer that provides lowionic resistance but offers poorer chemical resistance thus producing amultilayer composite that meets both the three primary separatorrequirements and secondary chemical resistance requirements. Similarly,materials with very low ionic resistance but low tensile strength can becombined in a thick layer with a thin layer that provide good tensilestrength but offer higher ionic resistance thus also producing amultilayer composite that meets both the three primary separatorrequirements and secondary chemical resistance requirements.

When designing a multi-layered separator of this invention to satisfythe primary properties, there are three expressions that need to besatisfied by the composite multilayer membranes.

For ionic resistance the following equation must be satisfied

$R \geq {\frac{1}{A}{\sum\limits_{i = 1}^{n \geq 2}\; {\rho_{i}t_{i}}}}$

R is the ionic resistance specification for the film as determined bythe application requirements using the following equation R=V/I a whereV is the maximum voltage drop suitable for the application at themaximum drain rate of the application I and “a” is the area of the cell.“A” is the total area of the test sample typically 1 cm² if R is to beexpressed in units of Ω/cm². “n” is the number of layers which isgreater than or equal to two, ρ_(i) is the ionic resistivity (units ofohm cm) of the i^(th) layer in the composite and t_(i) is the thicknessof the i^(th) layer.

For electrical resistance the following equation must be satisfied

$R_{elec} \leq {\frac{1}{A}{\sum\limits_{i = 1}^{n \geq 2}\; {r_{i}t_{i}}}}$

R_(elec) is the electrical resistance specification for the film asdetermined by the application requirements using the following equationR_(elec)=V_(cell)/I_(self) a where V_(cell) is the cell open circuitvoltage, I_(self) is the maximum self discharge rate for the applicationand “a” is the area of the cell. “A” is the total area of the testsample typically 1 cm² if R is to be expressed in units of Ω/cm². “n” isthe number of layers which is greater than or equal to two, r_(i) is theelectrical resistivity (units of ohm cm) of the i^(th) layer in thecomposite and t_(i) is the thickness of the i^(th) layer.

For tensile strength the following equation must be satisfied

$S \leq {L{\sum\limits_{i = 1}^{n \geq 2}\; {\sigma_{i}t_{i}}}}$

S is the total Newtons of force per length of film normal to the force(L) at break. “n” is the number of layers which is greater than or equalto two, σ_(i) is the tensile strength (units of Newtons per cm²) of thei^(th) layer in the composite and t_(i) is the thickness of the i^(th)layer.

In other embodiments, the multilayer separators of this invention alsoprovide advantages for electrode shape change. Many electrodes producesoluble species that can migrate and precipitate in parts of the cellthat are not proximal to the electrode. This is one factor that drivesshape change in zinc electrode for example. Silver electrodes also havesoluble intermediaries that if allowed to freely migrate away from thesilver electrode the severely limit the cycle life of the cell.Multilayer separators that satisfy the three primary separatorrequirements that also further satisfy specific transport properties cangreatly reduce migration of soluble electrode species. The relativetransport properties of the individual layers of the multilayerseparators are derived as a time dependent, transient, diffusionproblem. For this problem, it is possible to set up a situation wherethe concentration diffusing ion at the boundary between two layers makesdiscontinues steps in concentration. A concentration step between twolayers apparently violates Fick's Second Law of diffusion. However,Fick's Second Law is a generalization where concentration is the maindriving force for diffusion [A. N. Malakhov and A. L. Mladentsev,“Nonstationary Diffusion in a Multiphase Medium”, Radiophysics andQuantum Electronics, Vol. 35, p 38-46, 1995]. In the more generalexpression the gradient in electrochemical potential is the drivingforce for diffusion. A generalized form for this equation is

$\frac{\partial{c( {x,t} )}}{\partial t} = {\frac{\partial\;}{\partial x}\lbrack {{D(x)}\frac{\partial{\mu ( {x,t} )}}{\partial x}} \rbrack}$

Where C(x,t) is the concentration of the diffusing species at position xand at time t, D is the effective diffusion (or diffusion/migration)constant and μ(x,t) is the electrochemical potential of the diffusingspecies. The definition of electrochemical potential is given by thefollowing equation

μ=μ°+RT ln(a)+zFΦ

Where μ° is the chemical potential at the reference conditions, R is theideal gas constant and T the absolute temperature, “a” is the activityof the diffusing species, z is the charge on the ion, F is Faraday'sconstant, and Φ is the electrostatic potential. The following commonapproximate expression for activity

a=γC

Where γ is the activity coefficient and C the concentration of diffusingspecies. For two layers of separator the electrochemical potential andtherefore the activity, neglecting the electric field, across theinterface should be continuous. In the case for small electric fielddifference between the two layers setting the activities equal acrossthe interface give this equation where the 1 and 2 refer to the firstlayer and second layer of the separator.

$\frac{\gamma_{1}}{\gamma_{2}} = {\frac{C_{2}}{C_{1}}.}$

This last equation implies that to produce a 10% discontinuity inconcentration across the interface, the activity coefficients of thediffusing species need to differ by 10%. Activity differences greaterthan or equal to 10% are desired to produce beneficial discontinuitiesin concentration. Some dendrite forming materials may see benefits fromdiscontinuities less than 10% where others may require discontinuitiesmore than 10%.

Multilayer separators can also provide advantages for dendritepenetration resistance. Examples of materials that form metallicdendrites are zinc, silver, copper, lithium, and bismuth.

Dendrite penetration is a major cause of early cycle life failure formany chemistries of electrochemical cells.

Co-extruded multilayer separators of this invention help to decreasedendrite penetration. The estimation of the maximum velocity of agrowing dendrite is

$v_{\max} = {\lbrack \frac{F^{2}{Dc}_{\infty}}{8\gamma \; {RT}} \rbrack \eta^{2}}$

Where F is Faraday's constant, R is the ideal gas constant, t is theabsolute temperature and is the overpotential. γ is the surface energyof the dendrite material, D is the effective diffusion (ordiffusion/migration) constant of the ion plating to form the dendriteand C_(∞) is the concentration of ion around the dendrite tip. Thisexpression holds for a dendrite growing in an isotropic medium. When thedendrite approaches the boundary between two layers of separator wherethe second layer has either a lower effective diffusion (ordiffusion/migration) constant for the plating species or lowerconcentration of the plating species the plating rate in the dendritegrowth direction (z-direction) decreases by the proportional amount.This causes the dendrite to grow faster in the directions perpendicularto the previous direction of growth (x,y-direction) thus blunting thedendrite and causing it to grow parallel to the layer interface. In someembodiments, multilayer separator of this invention which resistdendrite growth possess the following property: the product of D C_(∞)in a first layer is at least 20% different from the product of D C_(∞)in an adjacent second layer. At lower overportentials and with materialswith higher surface energies the step in the product DC_(∞) could beless than 20% between two layers to slow dendrite penetration.

IV. ELECTROCHEMICAL CELLS

Another aspect of the present invention provides an electrochemical cellcomprising a cathode, an anode, an electrolyte, and a separator, asdescribed above. In electrochemical cells of the present invention, anysuitable cathode, anode, and electrolyte can be used.

A. Electrodes

Another aspect of the present invention provides electrochemical cellscomprising an alkaline electrolyte, a cathode, and an anode; wherein thecathode comprises a first active material and a first binder material;the anode comprises a second active material and a second bindermaterial. In several examples, the first binder material, the secondbinder material, or both comprises PVDF or PVDF copolymer.

In several embodiments, the cathode comprises at least 90 wt % of thefirst active material. For example, the cathode comprises at least 90 wt% of an active material selected from AgO, Ag₂O, HgO, Hg₂O, CuO, CdO,NiOOH, Pb₂O₄, PbO₂, LiFePO₄, Li₃V₂(PO₄)₃, V₆O₁₃, V₂O₅, Fe₃O₄, Fe₂O₃,MnO₂, LiCoO₂, LiNiO₂, or LiMn₂O₄.

In several examples, the active material of the cathode comprises AgO.In other examples, the AgO is doped with up to 10 wt % of Pb. In severalexamples, the AgO is doped with up to 5 wt % of Pb, or the AgO is dopedwith up to 5 wt % of Pb and is coated with up to 5 wt % Pb. Othersuitable silver oxide-type active materials include Ag₂O or Ag₂O₃, whichmay be used in combination with AgO and/or in combination with eachother.

In several embodiments, a cathode comprises up to about 10 wt % (e.g.,up to about 6 wt %) of a binder material. For instance, the cathodecomprises up to about 10 wt % of a binder that comprises PVDF or PVDFcopolymer. In other examples, the binder material comprises a PVDFcopolymer such as PVDF-co-HFP copolymer. In several embodiments, thePVDF-co-HFP copolymer has a mean molecular weight of less than about600,000 amu (e.g., less than about 500,000 amu, or about 400,000 amu).

In alternative embodiments, an anode useful in the presentelectrochemical cells comprises at least 90 wt % of the second activematerial. For instance, an anode comprises at least about 90 wt % of anactive material selected from Si, Sn, Al, Ti, Mg, Fe, Bi, Zn, Sb, Ni,Pb, Li, Zr, Hg, Cd, Cu, LiC₆, mischmetals, or oxides thereof. In severalexamples, the anode comprises an active material comprising Zn or ZnO.

In several embodiments, the anode comprises up to 10 wt % of a bindermaterial. For instance, the anode comprises up to 6 wt % of a bindermaterial. In several examples, the anode comprises binder materialcomprises up to 10 wt % of a binder material comprising. PVDF or PVDFcopolymer. For instance, the binder material comprises a PVDF copolymersuch as PVDF-co-HFP copolymer. In other examples, the PVDF-co-HFPcopolymer has a mean molecular weight of less than about 600,000 amu(e.g., less than about 500,000 amu, or about 400,000 amu).

B. Electrolytes

Electrochemical cells of the present invention comprise an alkalineelectrolyte. In several embodiments, the electrolyte comprises NaOH orKOH. For instance, the electrolyte can comprise aqueous NaOH or KOH, orNaOH or KOH mixtures with liquids substantially free of water, such asliquid polymers. Exemplary alkaline polymer electrolytes include,without limitation, 90 wt % PEG-200 and 10 wt % KOH, 50 wt % PEG-200 and50 wt % KOH; PEG-dimethyl ether that is saturated with KOH; PEG-dimethylether and 33 wt % KOH; PEG-dimethyl ether and 11 wt % KOH; andPEG-dimethyl ether (mean molecular weight of 500 amu) and 33 wt % KOH,that is further diluted to 11 wt % KOH with PEG-dimethyl ether having amean molecular weight of 200 amu.

Exemplary electrolytes include aqueous metal-hydroxides such as NaOHand/or KOH. Other exemplary electrolytes include mixtures of a metalhydroxide and a polymer that is liquid at a range of operating and/orstorage temperatures for the electrochemical cell into which itemployed.

In other embodiments, the electrolyte is an aqueous mixture of NaOH orKOH having a concentration of at least 8 M.

Polymers useful for formulating an electrolyte of the present inventionare also at least substantially miscible with an alkaline agent. In oneembodiment, the polymer is at least substantially miscible with thealkaline agent over a range of temperatures that at least includes theoperating and storage temperatures of the electrochemical device inwhich the mixture is used. For example, the polymer is at leastsubstantially miscible, e.g., substantially miscible with the alkalineagent at a temperature of at least −40° C. In other examples, thepolymer is liquid at a temperature of at least −30° C. (e.g., at least−20° C., at least −10° C., or from about −40° C. to about 70° C.). Inanother embodiment, the polymer is at least substantially miscible withthe alkaline agent at a temperature from about −20° C. to about 60° C.For example, the polymer is at least substantially miscible with thealkaline agent at a temperature of from about −10° C. to about 60° C.

In several embodiments, the polymer can combine with the alkaline agentat a temperature in the range of temperatures of the operation of theelectrochemical device in which is it stored to form a solution.

In one embodiment, the electrolyte comprises a polymer of formula (I):

wherein each of R₁, R₂, R₃, and R₄ is independently(V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), each of V₁, V₂, and V₃, is independently a bondor —O—, each of Q₁, Q₂, and Q₃, is independently a bond, hydrogen, or aC₁₋₄ linear unsubstituted alkyl, n is 1-5, and p is a positive integerof sufficient value such that the polymer of formula (I) has a totalmolecular weight of less than 10,000 amu (e.g., less than about 5000amu, less than about 3000 amu, from about 50 amu to about 2000 amu, orfrom about 100 amu to about 1000 amu) and an alkaline agent.

In several embodiments, the polymer is straight or branched. Forexample, the polymer is straight. In other embodiments, R₁ isindependently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁, Q₁,V₂, Q₂, and V₃ is a bond, and Q₃ is hydrogen. In some embodiments, R₄ isindependently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁, Q₁,V₂, Q₂, and V₃, is a bond, and Q₃ is hydrogen. In other embodiments,both of R₁ and R₄ are (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), each n is 1, each of V₁,Q₁, V₂, Q₂, and V₃ is a bond, and each Q₃ is hydrogen.

However, in other embodiments, R₁ is independently(V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁, Q₁, V₂, Q₂, and V₃is a bond, and Q₃ is —CH₃, —CH₂CH₃, —CH₂CH₂CH₃, or H. For example, R₁ isindependently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁, Q₁,V₂, Q₂, and V₃ is a bond, and Q₃ is —CH₃ or H.

In another example, R₁ is independently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), whereinn is 1, one of Q₁ or Q₂ is —CH₂—, —CH₂CH₂—, or —CH₂CH₂CH₂—; V₁ and V₂are each a bond; V₃ is —O—, and Q₃ is H.

In several other examples R₄ is independently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n),wherein n is 1, each of V₁, Q₁, V₂, Q₂, is a bond, and V₃ is —O— or abond, and Q₃ is hydrogen, —CH₃, —CH₂CH₃, or —CH₂CH₂CH₃. For example, R₄is independently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁,Q₁, V₂, Q₂, and V₃ is a bond, and Q₃ is —H, —CH₃, —CH₂CH₃, or—CH₂CH₂CH₃.

In another embodiment, R₁ is (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1,each of V₁, Q₁, V₂, Q₂, and V₃ is a bond, and Q₃ is —CH₃, and R₄ is(V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, each of V₁, Q₁, V₂, Q₂, is abond, and V₃ is —O—, and Q₃ is —H.

In some embodiments, R₂ is independently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n),wherein n is 1, each of V₁, Q₁, V₂, Q₂, and V₃ is a bond, and Q₃ is—CH₃, —CH₂CH₃, —CH₂CH₂CH₃, or H. In other embodiments, R₂ isindependently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, one of V₁, Q₁,V₂, Q_(z), and V₃ is —O—, and Q₃ is —H.

In some embodiments, R₃ is independently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n),wherein n is 1, each of V₁, Q₁, V₂, Q₂, and V₃ is a bond, and Q₃ is—CH₃, —CH₂CH₃, —CH₂CH₂CH₃, or H. In other embodiments, R₃ isindependently (V₁-Q₁-V₂-Q₂-V₃-Q₃)_(n), wherein n is 1, one of V₁, Q₁,V₂, Q₂, and V₃ is —O—, and Q₃ is —H.

In some embodiments, the polymer comprises a polyethylene oxide. Inother examples, the polymer comprises a polyethylene oxide selected frompolyethylene glycol, polypropylene glycol, polybutylene glycol,alkyl-polyethylene glycol, alkyl-polypropylene glycol,alkyl-polybutylene glycol, and any combination thereof.

In another embodiment, the polymer is a polyethylene oxide having amolecular weight or mean molecular weight of less than 10,000 amu (e.g.,less than 5000 amu, or from about 100 amu to about 1000 amu). In otherembodiments, the polymer comprises polyethylene glycol.

Alkaline agents useful in the electrolyte of the present invention arecapable of producing hydroxyl ions when mixed with an aqueous or polarsolvent such as water and/or a liquid polymer.

In some embodiments, the alkaline agent comprises LiOH, NaOH, KOH, CsOH,RbOH, or combinations thereof. For example, the alkaline agent comprisesLiOH, NaOH, KOH, or combinations thereof. In another example, thealkaline agent comprises KOH.

In several exemplary embodiments, the electrolyte of the presentinvention comprises a liquid polymer of formula (I) and an alkalineagent comprising LiOH, NaOH, KOH, CsOH, RbOH, or combinations thereof.In other exemplary embodiments, the electrolyte comprises a liquidpolymer comprising a polyethylene oxide; and an alkaline agentcomprising LiOH, NaOH, KOH, CsOH, RbOH, or combinations thereof. Forexample, the electrolyte comprises a polymer comprising a polyethyleneoxide and an alkaline agent comprising KOH.

In several exemplary embodiments, the electrolyte of the presentinvention comprises more than about 1 wt % of alkaline agent (e.g., morethan about 5 wt % of alkaline agent, or from about 5 wt % to about 76 wt% of alkaline agent). In one example, the electrolyte comprises a liquidpolymer comprising a polyethylene oxide and 3 wt % or more (e.g., 4 wt %or more, from about 4 wt % to about 33 wt %, or from about 5 wt % toabout 15 wt %) of an alkaline agent. For instance, the electrolytecomprises polyethylene oxide and 5 wt % or more of KOH. In anotherexample, the electrolyte consists essentially of a polyethylene oxidehaving a molecular weight or mean molecular weight from about 100 amu toabout 1000 amu and 5 wt % or more of KOH.

Electrolytes of the present invention can be substantially free ofwater. In several embodiments, the electrolyte comprises water in anamount of about 60% of the wt of the alkaline agent or less (e.g., about50% of the wt of the alkaline agent or less, about 40% of the wt of thealkaline agent or less, about 30% of the wt of the alkaline agent orless, about 25% of the wt of the alkaline agent or less, about 20% ofthe wt of the alkaline agent or less, or about 10% of the wt of thealkaline agent or less).

Exemplary alkaline polymer electrolytes include, without limitation, 90wt % PEG-200 and 10 wt % KOH, 50 wt % PEG-200 and 50 wt % KOH;PEG-dimethyl ether that is saturated with KOH; PEG-dimethyl ether and 33wt % KOH; PEG-dimethyl ether and 11 wt % KOH; and PEG-dimethyl ether(mean molecular weight of 500 amu) and 33 wt % KOH, that is furtherdiluted to 11 wt % KOH with PEG-dimethyl ether having a mean molecularweight of 200 amu.

In another embodiment, the electrolyte is aq. KOH having a concentrationof from about 10 M to about 18 M.

In another embodiment, the alkaline electrolyte is an aqueous solutioncomprising a hydroxide of an alkali metal selected from the groupconsisting of potassium, sodium, lithium, rubidium, cesium, and mixturesthereof. The hydroxide concentration is in the molar concentration rangefrom 4 M to 16 M (e.g., from about 8 M to about 16 M, or from about 10 Mto about 16 M). In one example, wherein the electrochemical cell is azinc-silver oxide battery, a the electrolyte is 15 M potassiumhydroxide. The electrolyte can further comprise a gelling agent,polyethylene oxide, polyvinyl alcohol, carboxyalkyl cellulose,polyacylonitrile, polyacrylic acid, polymethacrylic acid, polyoxazoline,polyvinylpyrrolidine, polyacrylate or polymethacrylate, for example.

IV. METHODS

The present invention also provides methods of producing a separator ofthe present invention comprising providing a PE polymer mixture, asdescribed above, and providing a PVA polymer mixture as described above,wherein the PE polymer mixture and the PVA polymer mixture are providedto form a unitary separator comprising a PE polymer layer as describedabove and a PVA polymer layer as described above.

V. EXAMPLES Example No. 1 Exemplary Cells A and B

The efficacy of the invention was demonstrated for a bi-functionalseparator comprising a polyvinyl alcohol (PVA) layer and a zirconiumoxide-polyethylene oxide (ZrO₂—PEO) layer. For comparison, zinc-silveroxide test cells incorporating the bi-functional separator (Cell B) andthose incorporating individual separator layers of the same compositionas the bi-functional separator layers (Cell A) were evaluated viacharge-discharge cycle testing.

Solutions having substantially equivalent compositions were used toprepare the bi-functional separator and the individual separator layers.Polyvinyl alcohol layers were deposited from a 10 wt % PVA solution. ThePEO solution comprised 87 to 97 wt % water, 2 to 6 wt % polyethyleneoxide, 2 to 6 wt % yttria-stabilized zirconium oxide (filler), 0.2 to1.5 wt % potassium titanate (conductivity enhancer), and 0.08 to 0.2 wt% Triton X-100 (surfactant). Conventional dispersing techniques wereused to provide a uniform dispersion of the filler.

The bi-functional separator was prepared by co-extrusion of the PVA andPEO solutions from a two-layer slot-die unit, and drying at 280° C. Theindividual separator layers were prepared using conventional filmcasting techniques.

FIG. 4 illustrates the electrode-separator configuration used for thetest cells, which comprised three cathodes (111, 113 and 115) and twoanodes (112 and 114). Cathode 113 was two-sided, being sandwichedbetween anodes 112 and 114, and had the same capacity as the twoone-sided cathodes (111 and 115) combined. The anodes comprised a totalof 14 grams of zinc, and the cathodes comprised a total of 22 grams ofsilver. Electrodes 111, 112, 113, 114 and 115 were wrapped in individualSolupor separator films (DSM Solutech) 121, 122, 123, 124 and 125,respectively, and were charged with 40 wt % aqueous potassium hydroxideelectrolyte under a reduced pressure chamber before being incorporatedin cells. The solution uptake ranged from 10 to 20 wt % for the anodesand 15 to 25% for the cathodes. Note that the Solupor films are passiveseparators in that they function primarily as electrolyte reservoirs.

As depicted in FIG. 4, an active separator stack 131 was serpentinedback and forth between the electrodes. Active separator stack comprisedeither two bi-functional PVA/ZrO₂—PEO separator layers according to theinvention, or a ZrO₂—PEO layer and two PVA layers. In both cases, anoxidation-resistant ZrO₂—PEO layer faced the cathode, and adendrite-resistant PVA layer faced the anode. The two bi-functionallayers had a total thickness of 60 μm, compared to 70 μm for the threeindividual layers. After being assembled, each cell was charged with anadditional 0.25 mL of 40 wt % KOH solution, and was vacuum-sealed in apolyethylene bag for cycle testing. Cycle tests involved discharge at950 mA and charge at 690 mA.

FIG. 5 shows plots of charge capacity versus cycle number for twozinc-silver oxide cells employing a ZrO₂—PEO separator layer and two PVAseparator layers. Both cells exhibited an appreciable loss in capacityafter about 50 cycles and failed in less than 100 cycles.

FIG. 6 shows plots of charge capacity versus cycle number for twozinc-silver oxide cells (A and B) employing two bi-functionalPVA/ZrO₂—PEO separator layers. Capacity loss for both of these cells aspractically negligible after more than 125 cycles.

Example 2 Multi-Layer Separators

The following separator materials are useful for constructing separatorsof this invention.

Sample Code T1 is PVA/V6/PSS, where PVA is about 10 wt % PVA; V6 isabout ˜10 wt % PVA & ZrO₂ (˜35 wt % vs. PVA); and PSS is polystyrenesulfonic acid 25 wt % commercial PSS solution (Mw=1M). The separatorfilm was cast and dried overnight at ambient conditions.

Sample Code T2 is PVA/V6/(PSS+PAA), where is about 10 wt % PVA; V6 isdescribed above; PSS+PAA is (35 wt % PAA vs. PSS) solution was preparedusing PSS resin (Mw=1M) and a 25 wt % commercial PAA solution (192058Aldrich Poly(acrylic acid) partial sodium salt solution average Mw˜240,000 by GPC, 25 wt. % in H2O); film was cast and dried at low dryertemperatures.

Sample Code T3 is V6/PVA/PSS+PAA, where V6 is described above; PVA is 10wt % PVA; and PSS+PAA is (10 wt % PAA vs. PSS) solution was prepared bydilution 25 wt % PSS solution (Mw=1M) to 12.5 wt % and to it was added a25 wt % commercial PAA solution (192058 Aldrich Poly(acrylic acid)partial sodium salt solution average Mw ˜240,000 by GPC, 25 wt. % inH2O) to achieve a 10:1 PSS:PAA solid concentration; film was cast anddried at low dryer temperature.

Sample Code T4 is PSS+PAA (10%). 6.5 wt % PSS prepared from resin(Mw=1M); to the PSS solution was added 10 wt % PAA (vs. PSS resin) insolution form (192058 Aldrich Poly(acrylic acid) partial sodium saltsolution average Mw ˜240,000 by GPC, 25 wt. % in H2O). film was cast anddried overnight at ambient condition.

Sample Code T5 is PSS+PAA (20%). 6.5 wt % PSS prepared from resin(Mw=1M); to the PSS solution was added 20 wt % PAA (vs. PSS resin) insolution form (192058 Aldrich Poly(acrylic acid) partial sodium saltsolution average Mw ˜240,000 by GPC, 25 wt. % in H2O). film was cast anddried overnight at ambient condition

Sample Code T6 is PSS+PSS-co-MA(1:1) (20%). MA is malaic anhydride/acidwith PSS.

Sample Code T7 is PSS+PAA (35%). 6.5 wt % PSS prepared from resin(Mw=1M); to the PSS solution was added 35 wt % PAA (vs. PSS resin) insolution form (192058 Aldrich Poly(acrylic acid) partial sodium saltsolution average Mw ˜240,000 by GPC, 25 wt. % in H2O). film was cast anddried overnight at ambient conditions.

Sample Code T8 is PVA/T7/T7. PVA is 10% PVA bottom layers with two T7layers, where T7 is described above.

Sample Code P1 is PVA+PSS (10 wt % vs PVA). PVA is a (10% stock)solution mixed with PSS solution (20% stock) to provide a 10 wt % PSS.

Sample Code P2 is PVA+PSS (20 wt % vs PVA). PVA is a (10% stock)solution mixed with PSS solution (20% stock) to provide a 20 wt % PSS.

Sample Code P3 is PVA+PSS (20 wt % vs PVA). PVA (9% stock) solutionmixed with PSS solution (20% stock) to provide a 20 wt % PSS

Sample Code T9a is P2/T7/T7. P2 and T7 are described above.

Sample Code T9b is P3/T7/T7. P3 was prepared with 9% PVA stock solution(PVA:PSS=8:2). T7 is described above.

Sample Code T10 (1012-59) is T7/PVA/T7, in which a 10% PVA layer issandwiched between two T7s.

Sample Code P4 is PVA(10%)+PSS (20% vs. PVA). PSS was obtained from acommercial 25 wt % solution.

Sample Code T11 is T7/P4/T7, in which T7 and P4 are described above.

Sample Code T10F3 is the same configuration as T10 in which each layeris 8 μm.

Other Embodiments

The preferred embodiments of the present invention have been illustratedand described above. Modifications and additional embodiments, however,will undoubtedly be apparent to those skilled in the art. Furthermore,equivalent elements can be substituted for those illustrated anddescribed herein, parts or connections might be reversed or otherwiseinterchanged, and certain features of the invention can be utilizedindependently of other features. Consequently, the exemplary embodimentsshould be considered illustrative, rather than inclusive, while theappended claims are more indicative of the full scope of the invention.

1. A method of producing a separator comprising: providing a PE polymermixture, and providing a PVA polymer mixture, wherein the PE polymermixture and the PVA polymer mixture are provided to form a unitaryseparator comprising a PE polymer layer and a PVA polymer layer, whereinthe PE polymer layer resists oxidation and the PVA polymer layer resistsdendrite formation.
 2. The method of claim 1, wherein the separator hasa total thickness of less than 200 microns.
 3. The method of claim 2,further comprising providing 1 to 10 additional polymer mixtures,wherein the polymer mixtures are provided to form a separator comprisinga PE polymer layer, a PVA polymer layer, and from 1 to 10 additionalpolymer layers.
 4. The method of claim 1, further comprising providing aporous substrate.
 5. The method of claim 2, wherein the PE polymermixture and the PVA polymer mixture are provided on the porous substrateto form a unitary separator.
 6. The method of claim 3, wherein the PEpolymer mixture and the PVA polymer mixture are provided on opposingsides of the porous substrate to form a unitary separator.
 7. The methodof claim 5, wherein the porous substrate comprises a polyolefinmaterial.
 8. The method of claim 7, wherein the porous substratecomprises polyethylene or polypropylene.
 9. The method of claim 1,wherein the PE polymer mixture and the PVA polymer mixture are eachprovided by coextrusion to form a unitary separator.
 10. The method ofclaim 9, wherein either of the PE polymer mixture is at least partiallycured before being provided with the PVA polymer mixture, or the PVApolymer mixture is at least partially cured before being provided withthe PE polymer mixture.
 11. A multi-functional separator comprising aporous substrate film, and a plurality of active separator layerswherein at least one of the active separator layers is deposited on thefilm.
 12. The multi-functional separator of claim 11, wherein a firstactive separator layer and a second active separator layer are depositedon opposite sides of the porous substrate film.
 13. An electrochemicalcell comprising an electrolyte, an anode, a cathode, and amulti-functional separator, wherein the electrolyte is an alkalineelectrolyte, the anode comprises zinc metal, and the multi-functionalseparator comprises: an oxidation-resistant separator layer depositedfrom a PE solution comprising a polyether polymer that can be linear orbranched and can be unsubstituted or substituted; and adendrite-resistant separator layer deposited from a PVA solutioncomprising a cross-linking agent and a polyvinyl alcohol precursorpolymer, which can be linear or branched and can be unsubstituted orsubstituted.
 14. The electrochemical cell of claim 13, wherein thealkaline electrolyte comprises an aqueous solution of a hydroxide of analkali metal selected from the group consisting of potassium, sodium,lithium, rubidium, cesium, and mixtures thereof.
 15. The electrochemicalcell of claim 13, wherein the cathode comprises an active materialselected from the group consisting of silver oxide, nickel oxide, cobaltoxide, and manganese oxide.
 16. The electrochemical cell of claim 13,wherein the polyether polymer comprises polyethylene oxide orpolypropylene oxide, or a copolymer or a mixture thereof.
 17. Theelectrochemical cell of claim 13, wherein the cross-linking agent isboric acid.
 18. The electrochemical cell of claim 13, wherein one orboth of the PE solution and the PVA solution further comprise a powderof a metallic oxide selected from the group consisting of zirconiumoxide, titanium oxide and aluminum oxide.
 19. The electrochemical cellof claim 13, wherein one or both of the PE solution and the PVA solutionfurther comprise a titanate salt of an alkali metal selected from thegroup consisting of potassium, sodium, lithium, rubidium, cesium, andmixtures thereof.
 20. The electrochemical cell of claim 13, wherein oneor both of the PE solution and the PVA solution further comprise asurfactant.
 21. The electrochemical cell of claim 13, wherein the PVAsolution further comprises a plasticizer.
 22. The electrochemical cellof claim 13, wherein the PVA solution further comprises a conductivityenhancer consisting of a coploymer of polyvinyl alcohol and ahydroxyl-conducting polymer selected from the group consisting ofpolyacrylates, polylactones, polysulfonates, polycarboxylates,polysulfates, polysarconates, polyamides, and polyamidosulfonates.
 23. Amulti-functional separator comprising at least three active separatorlayers, wherein the multi-functional separator has an ionic resistanceof <10 Ω/cm², electrical resistance of >10 kΩ/cm², and a wet tensilestrength of >0.1 lbf.
 24. The multi-functional separator of claim 23,wherein the ionic resistance is <0.5 Ω/cm2.
 25. The multi-functionalseparator of claim 24, wherein the least two of the three activeseparator layers comprise a polymeric material each individuallyselected from PVA and PSA, or combinations thereof.
 26. Themulti-functional separator of claim 25, wherein the PSA comprises PSS.27. The multi-functional separator of claim 26, wherein themulti-functional separator comprises the layers PVA/V6/PSS;PVAN6/(PSS+PAA); V6/PVA/(PSS+PAA); PVMPSS+PAA(35%))/(PSS+PAA(35%));(PSS+PAA(35%))/PVA/(PSS+PAA(35%)); or (PSS+PAA (35%))/(PVA(10%)+PSS (20%vs. PVA))/(PSS+PAA (35%)).
 28. The multi-functional separator of claim27, wherein the multi-functional separator comprises the layersPVA/V6/(PSS+PAA); V6/PVA/(PSS+PAA); or (PSS+PAA(35%))/PVA/(PSS+PAA(35%)).
 29. The multi-functional separator of claim 28, wherein theseparator thickness is <100 mm.
 30. The multi-functional separator ofclaim 29, wherein the separator thickness is <30 μm.
 31. Themulti-functional separator of claim 30, wherein each layer in theseparator is <10 μm.
 32. (canceled)
 33. The multi-functional separatorof claim 23, wherein at least two layers of the separator comprise apolymeric material each individually selected from PVA, a quaternaryammonium polymer, or combinations thereof.
 34. A method of producing aseparator comprising: providing a PSA polymer mixture, and providing aPVA polymer mixture, wherein the PSA polymer mixture and the PVA polymermixture are provided to form a unitary separator comprising a PSApolymer layer and a PVA polymer layer, wherein the PSA polymer layerresists oxidation and the PVA polymer layer resists dendrite formation.35. The method of claim 34, wherein the separator has a total thicknessof less than 100 microns.
 36. The method of claim 34, further comprisingproviding 1 to 10 additional polymer mixtures, wherein the polymermixtures are provided to form a separator comprising a PSA polymerlayer, a PVA polymer layer, and from 1 to 10 additional polymer layers.37. The method of claim 36, wherein the separator has an ionicresistance of <10 Ω/cm2, electrical resistance of >10 kΩ/cm², and a wettensile strength of >0.1 lbf.
 38. The method of claim 37, wherein theionic resistance is <0.5 Ω/cm².